Effect of Short-Term Treatment of Roots with Melatonin on Photosynthesis of Cucumber Leaves

Effect of a short-term (15, 30, 60, and 120 min) root treatment with melatonin (0.1 pM and 1.0 mu M) on physiological parameters of the leaf was studied in 21-day-old cucumber (Cucumis sativus L.) plants, early cv. Izyashchnyi, grown on Murashige-Skoog nutrient medium. Experimental data concerning kinetics of melatonin influence have shown that the duration of the aftereffect necessary for activation of photosynthesis, transpiration, and stomatal conductance of cucumber leaves decreased when the concentration of the exogenous melatonin used for root treatment rose. A similar pattern was observed in respect to intensification of lipids' peroxidation, content of proline, and activation of guaiacol-dependent peroxidase (EC 1.11.1.7). Under the effect of melatonin, antiradical activity in the leaf decreased in 120 min, which was preceded by a reduction in the content of total flavonoids. Regulation in activity of antioxidant enzymes superoxide dismutase (EC 1.15.1.1) and catalase (EC 1.11.1.6) was more intricate. Possible mechanisms of the influence of melatonin on parameters of photosynthesis in cucumber leaves are discussed.